--- old/src/hotspot/share/gc/shenandoah/shenandoahSharedVariables.hpp	2020-06-01 12:25:24.849026038 +0200
+++ new/src/hotspot/share/gc/shenandoah/shenandoahSharedVariables.hpp	2020-06-01 12:25:24.533026448 +0200
@@ -245,4 +245,38 @@
 
 };
 
+typedef struct ShenandoahSharedSemaphore {
+  shenandoah_padding(0);
+  volatile ShenandoahSharedValue value;
+  shenandoah_padding(1);
+
+  static uint max_tokens() {
+    return sizeof(ShenandoahSharedValue) * CHAR_MAX;
+  }
+
+  ShenandoahSharedSemaphore(uint tokens) {
+    assert(tokens <= max_tokens(), "sanity");
+    Atomic::release_store_fence(&value, (ShenandoahSharedValue)tokens);
+  }
+
+  bool try_acquire() {
+    while (true) {
+      ShenandoahSharedValue ov = Atomic::load_acquire(&value);
+      if (ov == 0) {
+        return false;
+      }
+      ShenandoahSharedValue nv = ov - 1;
+      if (Atomic::cmpxchg(&value, ov, nv) == ov) {
+        // successfully set
+        return true;
+      }
+    }
+  }
+
+  void claim_all() {
+    Atomic::release_store_fence(&value, (ShenandoahSharedValue)0);
+  }
+
+} ShenandoahSharedSemaphore;
+
 #endif // SHARE_GC_SHENANDOAH_SHENANDOAHSHAREDVARIABLES_HPP